Based on our extensive experience with Morris hepatomas plus new data on 122 autochthonous, slowly growing hepatomas, we feel justified in proposing the hypothesis that, simplistically defined, there is a class of "minimal deviation" hepatomas that may be distinguished from "progressed" hepatomas by their low capability to transport, i.e. "pump in", important nutrients with many consequential functions, e.g., amino acids and glucose. We have previously shown that among the available transplantable Morris hepatomas there is a wide diversity of amino acid transport capabilities. The more rapidly growing hepatomas had characteristic transport levels higher than that of liver, while a few hepatoma lines were slowly growing and had low transport levels. Our recent experience has shown that the slowly growing autochthonous hepatomas were like the slowly growing transplantable hepatomas in having low transport capability (Kelley and Potter, Biochem. Biophys. Res. Comm. 75: 219-225 (1977). The definition of the minimal enzymatic changes required for the conversion of a normal liver cell to a hepatoma remains our major objective, but new information as to approach and choice of parameters provides the possibility for a sharper focus on the problem. The new elements are: 1) The ability to produce slowly growing autochthonous hepatomas, 2) the ability to study amino acid and glucose transport in vivo and in cell cultures in conjunction with any other combination of parameters, and 3) the techniques for electrophoretic separation and autoradiographic detection of proteins labeled with tritium-containing amino acids.